Long distance optical transmission systems that employ optical relay amplifier transmission systems that use an erbium doped fiber amplifier (EDFA) that can directly amplify light of a 1.5 micrometer band have recently become a main trend. Further, recently, high capacity transmission systems using wavelength-division multiplexing transmission systems are realized by the EDFA that can amplify light in a wide band.
To realize a higher capacity transmission system at a lower cost, increase in a transmission speed per one channel, effective utilization of amplification band (narrowing of wavelength multiplexing interval), and lengthening of relay interval are indispensable.
However, when the transmission speed is increased and/or the relay interval is lengthened, an excessively high optical signal-to-noise ratio is required at the receiving end.
Difference phase shift keying (DPSK) modulation systems that do not have the problem are drawing attention. The difference phase shift keying (DPSK) modulation systems can double the reception sensitivity even at the optical signal-to-noise ratio that is used conventionally.
The DPSK modulation system differentially encodes a phase change between information data series generated from a binary on and off signals, and phase modulates a direct current light. For example, when there is no phase change between data (that is, when a phase change is zero), the DPSK modulation system generates an on signal as a differentially encoded signal. When there is a phase change between pulses (that is, when a phase change is π), the DPSK modulation system generates an off signal as a differentially encoded signal. Particularly, a system that executes a phase modulation of (0, π) based on on and off of the differentially encoded signals using a phase modulator is an NRZ (non-return-to-zero)-DPSK modulation system.
In DPSK modulation system, a transmitting terminal phase modulates a continuous wave (CW) light using a differentially encoded signal obtained by differentially encoding a phase change between information data series generated from the binary on and off signals.
On the other hand, a receiving terminal generates a differentially encoded signal from a DPSK signal, and decodes original data signal from the differentially encoded signal. More specifically, the receiving terminal includes a self delay interference detector that has a one-bit delay interferometer, two photodetectors, and a discriminator, and that extracts a data signal by executing a signal processing called self delay detection.
The self delay interference detector switches over between the two photodetectors depending on a phase of the result of interference determined by the one-bit delay interferometer. Specifically, when the phase difference of a detection signal detected by the one-bit delay interferometer is zero, one of the photodetectors processes the detection signal. When the phase difference is π, the other photodetector processes the detection signal. A signal processed by one of the photodetectors is an inverted output. Both detection signals are input to a latter-stage discriminator, and a data signal is detected. In other words, according to the self delay interference detector, the two photodetectors process detection signals depending on a phase of a result of the interference. Therefore, the self delay interference detector can achieve reception sensitivity two times that of the conventional on/off keying modulation (binary amplitude modulation) system.
As explained above, the DPSK modulation system that can achieve reception sensitivity two times that of the on/off keying modulation system used in the conventional optical transmission system has a possibility of achieving a long distance transmission in high-speed optical communications.
Patent documents 1 and 2 disclose optical transmission apparatuses that transmit a signal by using an RZ (return-to-zero)-DPSK signal which is obtained by intensity modulating the NRZ-DPSK signal into an RZ signal, in addition to the NRZ-DPSK modulation system.
These documents disclose examples of optical transmission apparatuses that use the RZ-DPSK modulation system, and also introduce several papers on the RZ-DPSK modulation system.
For example, the patent document 1 states that the following contents are described in a non-patent literature 1: “It is predicted by simulation that the RZ signal can have a reproduction relay distance expanded by about three times the distance obtained by the NRZ signal at 40 Gbits/s, in a linear relay system in which a 1.3 micrometer zero dispersion fiber transmission path is dispersion compensated for in each relay sector”.
The patent document 1 also states that the following contents are described in a non-patent literature 2: “It is indicated by experiments that the RZ signal can have power increased per one channel as compared with power obtained from the NRZ signal, in a 10-Gbits/s 8-wavelength WDM transmission system”.
A non-patent literature 3 reports that “A 5200-kilometer long-distance transmission is achieved by using the RZ-DPSK modulation system”.
These description contents are considered to be describing the influence of transmission effects using the RZ signal instead of the effects of using the DPSK modulation system. In any case, it is considered preferable to use the RZ signal for transmission in a high-speed optical transmission system.
Patent document 1: Japanese Patent Application Laid-Open No. 2000-106543
Patent document 2: Japanese Patent Application Laid-Open No. 2001-251250
Non-patent literature 1: D. Breuer et al, “Comparison of NRZ and RZ-Modulation Format for 40-Gbit/s TDM Standard-Fiber Systems”, IEEE Photon. Technol. Lett. Vol. 9 No. 3 pp. 398-400, 1997.
Non-patent literature 2: R. M. Jopson et al, “Evaluation of return-to-zero modulation for wavelength-division-multiplexed transmission over convention single-mode-fiber” R. M. Jopson et al, in Tech. Digest of Optical Fiber Comm. Conf. '98 FE1, p. 406-407, 1998.
Non-patent literature 3: B. Zhu et al, “Transmission of 3.2 Tb/s (80×42.7 Gb/s) over 5200 km of UltraWave fiber with 100-km dispersion-managed spans using RZ-DPSK format”, Technical Digest of ECOC2002, paper PD. 4.2, September 2002.
The optical transmission apparatuses disclosed in the patent and non-patent literatures generate the RZ signal in an optical area. To generate the RZ signal in the optical area means to handle the NRZ signal in an electric area.
On the other hand, in the opposite case, that is, when an RZ electric signal is handled in the electric area, according to the patent document 1, a band required in an electric circuit becomes two times as compared with when an NRZ electric signal is handled in the electric area, and it becomes difficult to increase the transmission speed.
However, generation of the RZ signal in the electric area has advantages in that a circuit scale of the optical transmitter can be reduced and that stability of the apparatus and cost superiority can be achieved.
For example, in the optical transmission apparatus that generates an optical RZ-DPSK signal using an electric NRZ-DPSK signal, the optical phase modulator first phase modulates the electric NRZ-DPSK signal, and an optical intensity modulator intensity modulates the phase modulated signal, thereby generating an optical RZ-DPSK signal, as explained in the patent document 2. In other words, this optical transmission apparatus requires an additional circuit (optical phase modulator, in this example) as compared with when only the optical intensity modulator is used to directly generate the optical RZ-DPSK signal from the electric RZ-DPSK signal.
In the actual manufacturing of the apparatus, it is necessary to carry out various kinds of controls such as a temperature control and a synchronization control in plural circuits and apparatuses having different temperature characteristics and stabilities. When the scale of the apparatus increases, man-hours and costs required to design these controls and to manufacture the apparatus increase.
The patent document 1 also points out the following issues. In the conventional system of directly amplifying the RZ electric signals, when a capacity coupling type driving circuit is used, a variation occurs in a DC level of a driving waveform due to a variation in a mark rate. Therefore, two times of an output dynamic range of the driving circuit is required. A control circuit that compensates for a bias point of the optical intensity modulator that varies due to the mark rate, with the mark rate is also necessary.
However, the issue relates to an RZ-OOK (on-off keying) transmission system before the DPSK transmission system is used. According to the RZ-DPSK transmission system, only a phase change between adjacent data has an information component. Therefore, the variation of the mark rate and the variation of the DC level are not as big a problem as pointed out.
The present invention has been achieved in the light of the problems. It is an object of the invention to provide an optical transmitter that carries out an optical transmission using an optical RZ-DPSK signal, wherein the optical transmitter carries out an optical modulation using an electric RZ-DPSK signal, thereby reducing a circuit scale, providing excellent stability, and reducing cost.